[GENERAL] why can't see the updated value after SPI_execute("update ....", false, 1);

[sunpeng]

First I use SPI_execute("update ....
  bool succ;
   SPI_connect();
    int ret = SPI_execute("update ....where uid = 1", false, 1);//later will
add error processing
    if (ret == SPI_OK_UPDATE && SPI_processed == 1) {
        succ = true;
   }
   SPI_finish();

Then I use SPI_execute("select .....where uid = 1", ....

    SPI_connect();
    int ret = SPI_execute("select .....where uid = 1", true, 1);//later will
add error processing
    if (ret == SPI_OK_SELECT && SPI_processed == 1) {
           HeapTuple tuple;
           tuple = SPI_tuptable->vals[0];
              ...
           datum = heap_getattr(tuple,attrno,SPI_tuptable->tupdesc,&isnull);
           ....
    }
the datum doesn't change, why ?